It is known that the use of physico-chemical processes is common to most treatments applied to various types of water and that these treatments essentially consist of:
clarification of surface water for consumption or for industry; PA1 clarification of municipal sewage, storm water or industrial waste water; PA1 decarbonization; PA1 removal of phosphates; PA1 etc. PA1 coagulation: a step of neutralization of the colloids using a metal salt, generally a trivalent iron or aluminium compound, in order to form a microfloc. PA1 flocculation: a step of agglomeration and growth of the microfloc. This agglomeration step takes place by virtue of the addition of a polyelectrolyte (or polymer) downstream of the coagulation step; PA1 settling: a step of separation of the floc from the interstitial water, causing the formation of sludge on the one hand, and of clarified water on the other hand. PA1 flocculation with a contacting mass, which has allowed the quality of the flocs to be improved, the volume of the reactors to be reduced and the clarification to be improved. This is because the microflocs of the coagulation have a greater chance of agglomerating and of growing as the reaction medium contains a high density of particles: the rate of floc formation is proportional to the number of free particles in the suspension; PA1 lamellar settling, carried out by introducing inclined plates or tubes in the settling tanks. This technology has made it possible to reduce the size of the settling tanks by from 50 to 70% PA1 1. recirculated presettled sludge: an example of this technique is described in FR-A-2,553,082; PA1 2. fine ballasts, such as microsand: an example of the use of this technique is described in FR-P-1,411,792 and in FR-A-2,627,704. PA1 the contacting mass is generated by the process, and is therefore available without any quantity limitation, depending on the requirements of the process; PA1 the contacting mass presents a very high specific surface area or spatial occupation because of its expanded structure and its low relative density; by way of example, 1 gram of flocculated sludge in one liter (average concentration in the reactor) occupies, after settling for approximately 5 minutes, a volume equal to 100 ml. PA1 This very high specific surface area or spatial occupation considerably increases the probability of contact between the flocs and the very fine particles, coagulated colloids and micro-organisms, and therefore of "trapping" this suspended matter very efficiently. PA1 accelerated flocculation, by virtue of the use of a contacting mass having a high specific surface area (or spatial occupation); PA1 increase in the settling velocities, resulting from the addition of ballast to the floc. PA1 for equivalent contacting mass (by weight), the ballast offers a contacting surface area or percentage of spatial occupation which is much less than the sludge. By way of example: PA1 in the case of "flocculation with sludge", the concentration in the reactor is approximately 1 g/l and the volume occupied by the sludge after five minutes of settling is approximately 10% of the initial volume; PA1 in the case of "flocculation with ballast (for example sand)", the ballast concentration in the reactor should reach at least 5 g/l, while the volume occupied by sludge after five minutes of settling is only approximately 1% of the initial volume; PA1 increasing the amount of ballast, desirable for obtaining a high contacting mass (and not for obtaining a high settling velocity), leads to an increase in the volume of sludge recirculated to the extracted-sludge treatment system, which treatment consists in separating the sludge from the sand so as to regenerate the latter. This operation is generally carried out by hydrocyclones supplied at high pressures, which operation therefore becomes very expensive from an energy consumption standpoint. In fact, and so as to limit the running costs, the volume of recirculated sludge is intentionally limited to between 5 and 10% of the volume treated and the ballast concentration in the reactor does not exceed 5 to 10 g/l: quite obviously this choice is incompatible with the possibility of optimizing the flocculation. PA1 various techniques are aimed at compensating for the deficit in the contacting mass resulting from the operating conditions described above, such as: PA1 the use of additional flocculation energy (figures ranging up to 100 times the conventional flocculation energy may be mentioned) or PA1 the use of even finer ballast particles, increasing the specific surface area (for example, particles having a diameter of between 10 and 50 .mu.m), is not conceivable, on the one hand, for energy cost reasons and, on the other hand, for reasons of difficulty in settling and in sand-floc separation. PA1 the system is sensitive to sudden pollution caused by lack of availability of binding sites on the ballast (the contacting mass is limited to a maximum of 5-10 g/l); PA1 the system has a lower performance with regard to so-called "sensitive" pollutants (helminth eggs, microorganisms, microparticles, traces of complex organic compounds, pesticides etc.); PA1 the low concentration of extracted sludge, resulting from the need to clean the ballast as fully as possible--this concentration is at least 10 times lower than that measured on apparatus using a sludge contacting mass--and frequently involving the installation of a complementary unit, downstream of the settling tank, for thickening the extracted sludge.
These types of physico-chemical treatments always comprise the following successive steps:
This coagulation step may be carried out in one or more steps;
Over the last thirty years or so, the state of the art relating to such a physico-chemical treatment has evolved considerably as a result of the appearance of two technologies:
The current technological trend is towards improving the flocculation conditions, which are key in determining the quality of the treated water and in obtaining high settling velocities.
At the present time, modern settling tanks use two types of contacting masses in the flocculation reactor:
The advantages and disadvantages of the two known techniques indicated above, of flocculation with a contacting mass, will now be explained.
1. Flocculation Using Sludge as the Contacting Mass
FIG. 1 of the appended drawing shows diagrammatically a physico-chemical treatment plant employing this technique. This figure shows diagrammatically, at A, the coagulation reactor, at B, the flocculator and, at C, the settling tank. These are plants well known to those skilled in the art and, under these conditions, they will not be described.
Thus, as may be seen in this FIG. 1, the contacting mass in the flocculation reactor B consists of the recirculation of part of the sludge which has settled in C. The recirculated sludge volume represents between 0.5 and 4% of the treated volume. The excess, concentrated sludge is extracted and removed. Their volume represents between 0.1 and 1% of the treated volume, depending on the treatments.
The advantages of this flocculation technique using recirculated presettled sludge as the contacting mass are the following:
The drawbacks and limitations of this technique involve the settling speeds obtained with densified sludges which are between 30% and 80% of the velocities obtained with ballast.
2. Flocculation Using a Ballast as the Contacting Mass
According to this technique, the contacting mass is obtained by adding, upstream of a flocculator, a fresh or recycled ballast after cleaning. The means making it possible to separate and regenerate the ballast which is to be recycled in the flocculator are means well known to those skilled in the art and, under these conditions, they will not be described.
When implementing this technique, the ballast generally consists of sand and the continuously extracted materials amount to approximately 5% of the volume of water treated by the settling tank; these extracted materials, laden with sludge coating the microsand, must be treated so as to regenerate the sand; the cleaned sand is subsequently reinjected upstream of the flocculator, at the front of the plant. The residue generated by this sand-ballast cleaning operation represents the excess sludge.
It will be noted that the existing ballast-type apparatus described in the literature, and especially in FR-P-1,411,792 and in FR-A-2,627,704, include a ballast-recycling step for obvious running-cost reasons. Moreover, in all the documents describing this technology it is specified that the ballast is always "cleaned", i.e. regenerated. This is because, the ballast, "coated" with the polymer, must have the maximum area of adhesion for the precipitation flocs produced chemically during coagulation. An effective physical cleaning is therefore indispensable for maximizing the binding area available.
The ballast is often sand, generally having a diameter of between 50 .mu.m and 150 .mu.m, usually called microsand.
The publication Journal Water SRT-AQUA, Vol. 41, No. 1, pp. 18-27, 1992 describes a curve relating the turbidity of the water produced to the diameter of the ballast particles, which demonstrates that this process becomes effective when the sand particles do not exceed 150 .mu.m, the results being even better with values of the order of 50 to 100 .mu.m.
It should be pointed out that the advantage of this technique of flocculation using a contacting mass consisting of a fine ballast essentially resides in the settling velocity, which may be from 20% to 200% greater than the velocities obtained by the flocculation processes using a contacting mass consisting of recirculated presettled sludge. Thus, when clarifying river water, the indicated velocities through the lamellar modules are between 25 and 50 m.sup.3 /m.sup.2.h, while the equipment implementing the flocculation process using sludge as the contacting mass is limited to velocities of between approximately 15 and 30 m.sup.3/ m.sup.2.h.
The essential drawbacks of this technique mainly stem from the fact that the ballast must provide two different functions:
These limitations or drawbacks can be imputed to the following characteristics:
In summary, the performance characteristics of flocculation with ballast are limited by three factors: